The present invention relates to telecommunications cables. It relates more particularly to cables containing optical fibers disposed in a tube made of a thermoplastic elastomer having flexible diol segments, and in particular a thermoplastic polyester elastomer.
An optical fiber cable is generally constituted by an outer sheath and metal or dielectric protective reinforcing elements together surrounding a plurality of modules, each comprising a plurality of fibers surrounded by a protective tube. The purpose of these tubes is to protect the fibers both mechanically and chemically, and they are also used for identifying the modules. For example, a 144-fiber cable comprises 12 tubes each containing 12 fibers, the tubes being assembled around a central carrier, and the entire assembly being surrounded by a sheath of polyethylene. The fibers are individually colored within each tube, and the tubes are themselves colored.
It is known to place optical fibers in tubes that are rigid and thick-walled, and that do not come directly into contact with the fiber. EP-A-0 769 711 describes such tubes. WO-A-96/23239 or EP-A-0 968 809 are other examples of thick rigid tubes of filled polyethylene/polypropylene copolymer or of a thermoplastic matrix containing stiffening elements. Nevertheless, those tubes suffer in particular from the drawback of not enabling the fibers to be accessed with the fingers unless special tools are used, and of not being very flexible.
To protect the optical fiber in a telecommunications cable, flexible tubes are also known that are extruded directly onto the fiber. This type of tube is then referred to as a xe2x80x9cskinxe2x80x9d.
A solution of that type is described in the Applicant""s European patent application 00 400 187.1 of Jan. 25, 2000 (published under the No. EP-A-1 024 382). That document states that it is known to use plasticized polyvinylchloride (PVC) for the skin, and some polyolefins; as a flexible covering for an optical fiber, that document proposes using a polyolefin-based thermoplastic elastomer material having a modulus of elasticity of about 500 megapascals (MPa) at ambient temperature, and a modulus of elasticity of about 1500 MPa at xe2x88x9240xc2x0 C. An example is ethylene propylene (EPR) copolymer. As also stated in that document, such a skin must have properties of elasticity, while nevertheless being capable of being torn in order to give access to the fiber it protects. A new problem that the invention sets out to solve is that of sticking during sheath manufacture.
The outer sheath is generally made by extruding polyethylene around modules assembled at molten polymer temperatures that can locally be as high as 150xc2x0 C. or 160xc2x0 C. Unfortunately, at a temperature above the melting point of the polymers used, the modules run the risk of sticking together, and that can reduce protection, while also making it more difficult or even impossible to access the fibers and to identify them.
The invention seeks to propose a protective skin for optical fibers that are brought together in modules to form cables, and that give access to fibers without using tools while also withstanding temperatures of at least 130xc2x0 C.
The invention solves these problems by means of a protective skin made of a thermoplastic elastomer having flexible diol segments.
The invention also provides a method of manufacturing such optical fiber modules. Finally, the invention provides an optical fiber cable containing such modules.
More precisely, the invention provides a telecommunications cable module comprising at least one optical fiber surrounded by a protective skin of thermoplastic elastomer having flexible diol segments, with a melting point greater than 130xc2x0 C. and initial resistance to tearing of less than 120 kilonewtons per meter (kN/m). In an embodiment, the thermoplastic elastomer is a TEEE, and of these, preferably, a polybutylene terephthalate glycol block copolymer.
In an aspect of the invention, the thermoplastic elastomer may include a filler which can be selected from: carbon black, silica, kaolin, alumina, clay, talc, chalk, magnesia, and titanium dioxide.
In another aspect of the invention, the thermoplastic elastomer includes a fire-retarding agent.
In an embodiment, the thermoplastic elastomer possesses hardness on the Shore D scale of less than 50. In another embodiment, the melting point of the skin material is at least 140xc2x0 C. In yet another embodiment, the initial resistance to tearing of the skin material is less than 60 kN/m. In yet another embodiment, the breaking elongation of the skin material lies in the range 50% to 300%. In yet another embodiment, the ultimate tensile strength of the skin material lies in the range 5 MPa to 15 MPa. Finally, in another embodiment, the hot fluidity index to the skin material is greater than or equal to 2 grams per ten minutes (g/10 min).
The invention also provides a method of manufacturing a telecommunications cable module of the invention, the method comprising the step of applying a protective skin of thermoplastic elastomer having flexible diol segments onto said at least one optical fiber. In an implementation, the protective skin is applied by extrusion.
Finally, the invention also provides a telecommunications cable including at least one module of the invention.